The present invention pertains to a device for controlling the production and flow of foam at low flow rates.
Small bubble foam generators are known, as described in my U.S. Pat. Nos. 3,811,660 and 4,400,220, which are hereby incorporated by reference. These generators produce foam with small bubble size at fairly high rates, which is useful for many applications, such as the use of foam for dust suppression. However, there are some applications in which my small bubble foam is needed at low flow rates.
In accordance with my U.S. Pat. No. 3,811,660, it is necessary to cause the air, water, and surfactant mixture to be subject to "substantial agitation" to produce small bubble foam. This process is performed by causing the mixture to flow at or above a minimum velocity through a pipe, hose or foamer (a unit having "tortuous passages"), or through a foamer as shown in my U.S. Pat. No. 4,207,202 which is hereby incorporated by reference.
Many applications for "small bubble" foam require very small flow rates of the foam. These rates may be less than 1/16 gallon per minute. The problem of producing very small flow rates of small bubble foam is two-fold. One is the requirement for metering such small quantities or air, water, and surfactant on a continuous basis, and the second is the requirement for providing "substantial agitation" through some foaming device.
A particular application in which small bubble foam is needed at low flow rates is called froth flotation. Froth flotation, or benefication as it is sometimes called, is a concentration process for separating the fine valuable minerals from their gangue impurities. To effect benefication, mineral-bearing ores are ground in water to form a mixture of mineral particles and non-mineral gangue particles. The resulting mixture (water, ore, mineral particles, and gangue particles) is conditioned with various chemicals including froth-producing compounds and agitated in flotation machines which introduce and disperse air in the form of bubbles throughout the pulp to liberate the mineral particles from the gangue particles. The bubbles collect at the surface of the pulp as a froth in which the valuable mineral particles are entrapped. The separated minerals are then either skimmed off or overflow with the froth to concentrate tanks, from which the minerals are then extracted for further processing.
There are many different flotation machines, but all require the formation of some type of air bubbles in the pulp. The size of the air pockets (bubbles) in the pulp is determined by many factors including the air pressure, hole size, agitation of the pulp, etc. In one type of machine, compressed air is introduced under or into the pulp by perforated pipes or by expelling the air through multihole plates or fine mesh screens.
It is desirable to have the air pockets as small as possible to more efficiently separate the valuable fine mineral particles from the non-mineral gangue particles. However, present commercial equipment cannot produce air pockets much less than 1/64 inch diameter (0.015"); rather, they normally produce much larger bubbles between 1/32 and 1/4 inch diameter.
I have found in my actual measurement that the small bubble foam produced by equipment constructed according to my U.S. Pat. Nos. 3,811,660 and 4,400,220 have bubbles from 50 to 200 micron diameter (0.05 to 0.2 mm) (0.002-0.008 inches) when first ejected from the foam generator. These bubbles exist in a matrix consisting of water and surfactant in the form of highly stressed films surrounding small pockets of air. When this foam is introduced into a tank containing a pulp consisting of ground ore containing fine mineral and non-mineral (gangue) particles, the water film of the mass of bubbles disperses into the water of the pulp, leaving each bubble as a pocket of air surrounded by water. This results in a mass of air pockets which forms a froth which is very effective in entrapping the mineral particles. Thus, by using my small bubble foam the efficiency of the flotation machines is greatly improved.
The density (weight per unit volume) of the water into which the very small air pockets are introduced varies with the number of air pockets per unit volume of water. Therefore, it is necessary to accurately control the amount of air in the form of small air pockets introduced into the flotation machines.